1. Field of the Invention
The present invention is directed generally to removing smoke, dust and fumes from the air using an electrostatic precipitator (ESP) having high voltage plates. More specifically, the invention is directed to limiting arc discharge energy and suppressing the arc noise of electric arcs that can occur from the ESP's high voltage plates.
2. Description of Related Art
A conventional two-stage ESP operates by ionizing and precipitating aerosols from the air. Air laden with particles first passes through an ionization section of the ESP, where the particles receive unipolar charges and become charged particles. The air and the charged particles then pass through a precipitation section of the ESP, which includes an alternating series of charged and grounded plates, also known as precipitating plates. The plates generate high electric field gradients, and electrical forces drive the charged particles toward those plates that have a polarity opposite to that of the charge on the particles. This allows the charged particles to be precipitated and removed from the air with a high collection efficiency. The precipitating plates toward which the charged particles move are also known as collection plates.
There are primarily two types of ionizers. In the first type, fine tungsten wire's seven to 10 mils in diameter are commonly used as high voltage, ionizing electrodes. In the second type, sharp pointed elements such as spiked stainless steel blades, or sharp needles, are used as high voltage, ionizing electrodes. When supplied with a sufficient voltage, an ionize can generate unipolar ions in a concentration that ranges from 10 million to 100 million ions per cubic centimeter of air. Some of these ions then impart charge to any particles passing through the ionizer. With such a high ion concentration, airborne particles passing through the ionization section are usually charged up to a saturation level within a fraction of a second. The saturation level of a particle generally depends on the surface area of the particle. This is because the ions charge up the particle by adhering to it, and the number of ions that can adhere to the particle is generally limited by the available surface area of the particle.
The collection section of the conventional two-stage electrostatic precipitator is typically composed of a plurality of parallel plates. Some of the plates are electrically connected to a high voltage and others of the plates are grounded, and the plates are positioned in an alternating sequence so that for each plate in the sequence, the plate or plates adjacent to it have the opposite polarity. For example, a high voltage plate in the middle of the sequence will have grounded plates adjacent to it. Metal tie rods and aluminum spacers are usually used to physically secure the high voltage plates and the grounded plates, and also to appropriately connect each plate to either a high voltage source or to a ground. Clearance holes are provided in the high voltage plates to prevent the grounded tie rods and aluminum spacers from touching the high voltage plates, and clearance holes are provided in the grounded plates to prevent tie rods and aluminum spacers that are electrically connected to the high voltage source from contacting the grounded plates. In this manner, all high voltage plates are connected to the same high voltage level, and all grounded plates are grounded. The electric potential between the high voltage level and ground is typically thousands of volts, and can be, for example, between about 3 kV and about 6 kV.
A properly designed ESP does not arc during normal operating conditions. However, high voltage arcing can occur between the high voltage, or charged, plates and the grounded plates when the spacing between the plates is effectively reduced. For example, the airspace between the plates can be narrowed by the accumulation of deposited fibers, dust particles, lint, or other types of contaminants in the airspace between the plates. When the spacing between plates becomes less than the electrical breakdown distance of air, a high voltage between the plates can create a path or arc through the air over which current flows from the charged plate to the grounded plate. This arc discharge current creates a `firecracker-like` noise.
Arcing at any particular location on the plates can be continuous until the precipitator is cleaned and the cause of arcing removed, e.g., contaminants are removed so that the effective spacing between the plates is greater than the electrical breakdown distance of the air. The loud noise that an arc generates can be unpleasant and sometimes intolerable to a user. ESP designs that reduce the arc noise are thus highly desirable in various applications such as use in residences, restaurants, meeting rooms, hospitals, etc.
U.S. Pat. No. 4,166,729 to Thompson, et al. discloses precipitating plates made of a rigid, non-conducting material that is coated with a layer of low conductance material, to suppress arc noise. Because voltage potential cannot be effectively transferred through materials having a low conductivity, and since the arc discharge current flows from a voltage source through the low conductance material before reaching an air gap traversed by the arc, any voltage that is dropped across the low conductance material is unavailable at the air gap. This voltage drop reduces an amount of energy traversing the air gap during an arc discharge, and thereby reduces the corresponding arc noise. Thus, an arc discharge at any particular point in the precipitator is isolated from the voltage source by an impedance, and this suppresses a noise level of the arc discharge. In particular, a spark discharge in an ESP can be treated as a capacitor discharge and the total plate area comprises the capacitor. Since a high voltage discharge happens in milliseconds, if the RC time constant of discharge is sufficiently long, for example, greater than a tenth of a second, the high voltage spark or arc discharge over an air gap between two plates can be suppressed or eliminated. The RC time constant equals the resistance (in Ohms) of the plates multiplied by the capacitance (in Farads) of the plates.
One serious disadvantage of the above approach is the high cost of making such coated resistive plates. Precipitating plates which are insulated by a highly resistive material that is coated with an outer layer of low conductivity material can be relatively labor intensive and expensive to manufacture. In contrast, all-aluminum precipitating plates are die-stamped from a coil of aluminum in an operation that is typically carried out in continuous, repetitive cycles that allow large quantities of plates to be manufactured very cost effectively. However, all-aluminum precipitating plates do not suppress arc noise.